Labanc's goal at 13:21 gave San Jose a 4-3 lead, but Jonathan Marchessault tied the game 4-4 with 47 seconds remaining in regulation before Barclay Goodrow won it at 18:19 of overtime.
"You have a game, it's right there for you and things happen and all of a sudden, you're down, you claw your way back in, you think you have a chance to win, and just a mistake and give them a chance in overtime," Golden Knights defenseman Nate Schmidt said. "That's all it takes, one chance in overtime."
Vegas had a 3-1 lead in the best-of-7 series before losing 5-2 in Game 5 and 2-1 in the second overtime in Game 6.
"It is a very frustrating feeling, but we have to remember it for next year," Pacioretty said.